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Molecular & Cellular Oncology

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Cancer cells activate damage-tolerant and error-prone DNA synthesis

Elizabeth Mutter-Rottmayer, Yanzhe Gao & Cyrus Vaziri

To cite this article: Elizabeth Mutter-Rottmayer, Yanzhe Gao & Cyrus Vaziri (2016) Cancer cellsactivate damage-tolerant and error-prone DNA synthesis, Molecular & Cellular Oncology, 3:6,e1225547, DOI: 10.1080/23723556.2016.1225547

To link to this article: https://doi.org/10.1080/23723556.2016.1225547

Accepted author version posted online: 19Sep 2016.Published online: 14 Oct 2016.

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AUTHOR’S VIEW

Cancer cells activate damage-tolerant and error-prone DNA synthesis

Elizabeth Mutter-Rottmayera,b, Yanzhe Gaoa, and Cyrus Vaziria

aDepartment of Pathology and Laboratory Medicine, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA; bCurriculum in Toxicology,University of North Carolina at Chapel Hill, Chapel Hill, NC, USA

ARTICLE HISTORYReceived 11 August 2016Revised 11 August 2016Accepted 12 August 2016

ABSTRACTTrans-lesion synthesis (TLS) is a DNA damage-tolerant and error-prone mode of DNA replication. Recentwork shows that many cancer cells coopt an aberrantly expressed germ cell protein, melanoma antigen-A4 (MAGE-A4), to activate TLS. MAGE-A4–induced “pathological TLS” provides a potential mechanismthrough which neoplastic cells can tolerate intrinsic and therapeutic genotoxicity while acquiringmutability.

KEYWORDSCancer-testes antigens (CTA);DNA damage; genomemaintenance; melanomaantigen-A4 (MAGE-A4);mutagenesis; RAD18; trans-lesion synthesis (TLS)

Recent work suggests a new way in which some cancer cellsmay achieve DNA damage tolerance by hijacking an aberrantlyexpressed germ cell protein to activate trans-lesion synthesis(TLS). TLS is a post-replication repair mechanism that permitsongoing DNA synthesis in cells harboring damaged genomes,thereby sustaining cell proliferation and viability. When repli-cation forks encounter bulky DNA lesions, conventional repli-cative DNA polymerases are transiently replaced withspecialized DNA damage-tolerant “Y-family” DNA polymer-ases. Collectively, the Y-family TLS polymerases (POLH,POLK, POLI, and REV1) perform replicative bypass of diverseDNA lesions arising from environmental, intrinsic, and thera-peutic sources.1 In the absence of TLS, stalled replication forkscollapse and generate lethal DNA double-strand breaks (DSBs).However, Y-family polymerases are inherently error prone(particularly when replicating undamaged templates or bypass-ing non-cognate DNA lesions) and must be used in a limitedand tightly regulated manner to avoid mutagenesis.

Arguably, the best evidence that TLS can promote malig-nancy is provided by sunlight-sensitive and skin cancer-pronepatients with xeroderma pigmentosum-variant (XPV). Individ-uals with XPV have congenital defects in POLH, the TLS poly-merase responsible for error-free bypass of ultraviolet (UV)radiation-induced DNA lesions.2 UV-exposed XPV cells arehypermutable due to compensatory and error-prone bypass ofnon-cognate DNA lesions by alternative TLS polymeraseswhen POLH is absent or functionally inactive. Thus, replicationof UV-damaged DNA by the “wrong” DNA polymerases likelygenerates the mutations that drive skin carcinogenesis in XPV.However, with the exception of XPV, TLS polymerases are notconsidered to be dysfunctional in cancer. TLS is usuallyregarded as a housekeeping genome maintenance process in allsomatic cells. Whether altered TLS activity contributes more

broadly to tumorigenesis or whether TLS affects mutational sig-natures of cancer cells has not been addressed.

A new report shows that the TLS pathway is reprogrammedin many cancer cells, and may therefore play a more active rolein carcinogenesis than previously suspected. Gao and col-leagues defined a cancer cell-specific mechanism that sustainsDNA damage-tolerant trans-lesion synthesis by stabilizing anapical mediator of the TLS pathway, the E3 ubiquitin ligaseRAD18.3 RAD18 initiates TLS by mono-ubiquitinating prolif-erating cell nuclear antigen (PCNA), a DNA polymerase proc-essivity factor, and promotes recruitment of Y-family DNApolymerases to sites of DNA damage-induced replication forkstalling.4 Gao et al. identified the cancer/testes antigen (CTA)melanoma antigen-A4 (MAGE-A4) as a novel binding partnerand stabilizer of RAD18 in lung carcinoma cells.

MAGE-A4 is one of approximately 150 known CTAproteins that are typically germline restricted and absentfrom normal somatic cells but aberrantly overexpressed inmany different cancers.5 Members of the melanoma antigen(MAGE) family of CTAs share considerable structural simi-larity (particularly in the conserved core Winged-Helix A andWinged-Helix B domains).6 The MAGE proteins lack anyknown enzymatic activity and are therefore presumed tofunction as adaptors or mediators.

Although some CTAs have well-defined developmental rolesin the testes the contribution of MAGE proteins to normalgerm cell function is unknown. MAGE proteins have receivedattention because of their tumor-specific expression, primarilybecause they represent potential targets for cancer immunetherapy. However, several recent studies provide strong evi-dence that MAGEs (and some other CTAs) have biochemicalactivities that endow cancer cells with tumorigenic traits. Nota-bly, work by Potts and colleagues identified multiple MAGE

CONTACT Elizabeth Mutter-Rottmayer evmutter@email.unc.edu© 2016 Taylor & Francis Group, LLC

MOLECULAR & CELLULAR ONCOLOGY2016, VOL. 3, NO. 6, e1225547 (3 pages)http://dx.doi.org/10.1080/23723556.2016.1225547

family members as activating binding partners of specific E3really interesting new gene (RING) ubiquitin ligases.7 Thosestudies showed that the MAGE-C2–TRIM28 complex targetsp53 (also known as TP53) for degradation, thereby attenuatingimportant tumor-suppressive functions.7 More recently, thePotts group reported that MAGE-A3/6–TRIM28 ubiquitinatesand degrades AMPKa1 (or PRKAA1) and leads to inhibitionof autophagy.8 Given the emerging paradigm that MAGEsreprogram ubiquitin signaling networks in cancer cells, it isimportant to define the full repertoire of MAGE–E3 ligase com-plexes, identify their effector pathways, and test their roles incancer. The recent work by Gao et al.3 adds genome mainte-nance to the list of ubiquitin-mediated processes that arerewired by MAGEs in cancer cells. This study shows that sev-eral cancer cell lines rely on MAGE-A4 to maintain RAD18 lev-els, recruit POLH to UV-damaged chromatin, avertaccumulation of double-strand breaks, and resume DNA syn-thesis following UV irradiation. MAGE-A4–depleted cells reca-pitulate many hallmarks of TLS deficiency, consistent with aMAGE-A4–RAD18 signaling axis that sustains TLS in neoplas-tic cells.

What then is the putative selective advantage or tumorigenictrait conferred by CTA-induced pathological TLS in cancercells? Mutagenesis and DNA damage tolerance impact everyaspect of carcinogenesis (Fig. 1). For initiation of carcinogene-sis, cells must survive genotoxic stress, and permanently “fix”mutations via error-prone replication of damaged DNA. Pre-neoplastic and malignant cells often exist in harsh and stressfulDNA-damaging environments. How neoplastic cells toleratethe inherent stresses of tumorigenesis (including oncogene-induced replication stress and reactive oxygen species) is notwell understood. How DNA replication machinery is dysregu-lated to generate most of the mutations found in cancergenomes is also unknown. Cancer cells depend heavily on bothDNA damage tolerance and mutagenesis to survive, adapt, andresist chemotherapy. Thus the high-capacity TLS conferred byMAGE-A4–RAD18 signaling could potentially facilitate toler-ance of genotoxic and replicative stress. Additionally, excessiveand error-prone TLS due to MAGE-A4–RAD18 signaling rep-resents a potential mechanism for mutagenesis, a defining fea-ture and hallmark of cancer.9

Clearly, further work is needed to determine how MAGE-A4–RAD18 signaling influences the process of tumorigenesisand affects the genomic landscape of cancer cells. Indeed, adirect demonstration that any CTA affects tumorigenesis is stilllacking. In vivo experiments using defined mouse cancer mod-els are clearly necessary to test whether CTAs can contribute toinitiation, progression, chemoresistance, or maintenance oftumors.

Regardless of whether CTAs directly promote carcino-genesis, the many recent studies that identify CTA-inducedsignaling networks in established cancer cells3,7,10 may sug-gest new opportunities for highly selective targeted thera-pies. In this regard the MAGE-A4–RAD18 signaling axis isa very appealing therapeutic target pathway as neoplasticcells depend heavily on DNA damage tolerance and muta-genesis to survive, adapt, and resist therapy. In addition toits well-documented role in TLS, RAD18 activates severaladditional genome maintenance mechanisms including theFanconi anemia pathway and homologous recombination.4

Therefore, MAGE-A4 has the potential to stimulate toler-ance and repair of diverse DNA lesions (bulky adducts,DNA cross-links, DSBs) that are induced by anticanceragents. MAGE-A4–RAD18-mediated dependencies on DNAdamage tolerance and mutagenesis are vulnerabilities thatcould eventually be exploited to sensitize cancer cells tointrinsic or therapy-induced replicative stresses.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.

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Figure 1. Potential roles of MAGE-A4–driven trans-lesion synthesis in tumorigenesis. Melanoma antigen-A4 (MAGE-A4)-induced TLS provides a mechanism by which neo-plastic cells can tolerate carcinogenic exposures, oncogenic DNA replication stress, and therapy-induced DNA damage while promoting mutagenesis. The MAGE-A4–RAD18 signaling axis may provide opportunities for therapies that are highly specific for cancer cells.

e1225547-2 E. MUTTER-ROTTMAYER ET AL.

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